The invention relates to a dome-shaped pressurized can with a foil sack for holding a substrate which is positioned in the can cylinder. The invention also relates to a method for producing this type of pressurized can.
Pressurized cans with foil sacks are based on the principle of preventing the substrate contained in the foil sack from mixing with a pressurized gas which is necessary for ejecting the substrate from the pressurized can. This is intended to prevent impairment of the substrate properties and also provide a way to keep harmful propellants in the can or replace them with environmentally safe propellants. In the pressurized can according to the invention, the foil sack can contain a substrate made of a prepolymer with a foaming agent which is propelled from the can, forming a constructive polyurethane foam, with the aid of one of the environmentally safe gases instead of freon, which has often been used as the propellant but is now questionable for environmental reasons.
The invention is not limited to a particular substrate in the foil sack and can use the propellant suitable in each case. This is guaranteed primarily by the great strength of the junction between the dome and the cylinder in dome-shaped pressurized cans. This junction withstands a pressure of, for example, 24 bar. This junction is normally a multiple welt formed by the flange between the can cylinder, which can consist of a tin-plate sheet, and the groove of the dome, which can also consist of a tin-plate sheet, with these metal parts being folded over and underneath one another toward the outside. With present multiple welts, a flat gasket is placed on the groove base of the dome and incorporated into the external multiple welt. It withstands the pressure of the propellant used to eject the substrate from the can. In dome-shaped pressurized cans of this type, the propellant does not come into direct contact with the flat gasket. This prevents the propellant from entering the foil sack from above and mixing with the substrate.
The invention is not limited to a particular propellant. In particular, inert propellants such as carbon dioxide or nitrogen and solutions thereof can be used in addition to the propellants mentioned above, which are placed in liquid form between the foil sack and the pressurized can. The pressurized can according to the invention not only withstands the considerable pressure of the propellant but also prevents it from mixing with the substrate. The can also has the necessary junction strength between the foil sack and the multiple welt. Such stresses result from mechanical load on the junction, which is particularly high when the can is filled with the substrate through an opening in the dome, later to be closed by a valve, because the substrate is normally loaded through impact-producing means. Stress can also be produced by occasional pressure differences between the propellant chamber, which is located between the foil sack and the pressurized can, and the sack cavity, thereby subjecting the junction between the sack and the pressurized can to stress.
The high demands placed on the junction between the foil sack and the can welt have been taken into account through various means in known dome-shaped pressurized cans, although not to a sufficient degree. With multiple welts, for example, the top edge of the foil sack is known to be placed in the multiple welt, extending beyond the inner welt edge until it reaches a position between the metal parts of the welt. However, this has proved to overload the sack foil in the welt. The resulting foil deformations do not provide a sufficient seal between the propellant and the substrate in the foil sack. According to a different proposal, on the other hand, drawing the top edge of the foil sack over only the can flange can create the seal only by using additional, i.e. inserted, seals. This makes the manufacturing process considerably more difficult and expensive. Moreover, it cannot guarantee the mechanical strength of the sack junction.